This invention relates to a filter cloth assembly for use on a filter press plate, more particularly a center-feed filter press plate, which operates with a filter cloth on each side of the plate, a method for assembling a novel two-part filter cloth onto the plate, and a method for manufacturing the novel two-part filter cloth.
Filter presses are a well-known type of pressure filter made especially for liquid/solid separations. Such presses include a series of filter plates which are compressed together between two heads. Covering each plate is a filter cloth which covers all the area where filtration occurs. Each plate typically includes an opening located within the filtration surface area, usually near the center of the plate, and the plate has channels in its surface for passage of liquids. A solid/liquid mixture enters through the opening, passes into the spaces between the plates, then the liquid portion passes through the filter cloths into the channels and exits through an exit port for each plate. As the filtrate passes through the filter cloths, a cake of solids is built up in the spaces between the plates on the filter cloths. When the filter press is filled with solid filter cake material, the filtration is stopped, and the cake is either washed from the press or, alternatively, the press is opened, the plates separated, and the solid filter cake is removed. The filter cloth can then also be removed for washing or replacement.
In the center-hole type press, a conventional filter cloth passes through the center hole of the plate and covers both sides of the plate. Most of such filter cloths available for use are comprised of a single unit having two sides with an interconnected tubular center hole section, the two sides of filter cloth usually being of unitary construction, and typically sewn together. This construction, while typical, is barely manageable if the filter cloth is stiff in nature or large in size, because one side of the cloth must be folded together and squeezed through the center hole of the filter plate, then unfolded on the other side to cover the other side of the filter plate. This process is often very difficult and time consuming, particularly if the filter cloth must be changed frequently.
A metal, such as brass, or a plastic, screw-threaded, reusable mechanical coupling to hold two filter cloths together on opposite sides of a filter plate, has also been used. Each half has a flange which holds the filter cloth against the plate. Thread clogging with slurry and thread wear upon coupling and uncoupling the filter assembly can be problems. The amount of filter cloth caught behind and held by each flange is small, and hard turns of the coupling to hold it firmly in place under high pressure may damage the filter cloth. The amount of pressure used is also a variable with each instance of installation of the cloth by each installer. Further, threaded fittings may open up the interior of the filter plate/filter cloth assembly to bypass of the seal through the bottoms of the thread root channels under liquid pressure.
A filter cloth embodying two filter cloths is disclosed in U.S. Pat. No. 4,053,416. Two single pieces of filter cloth were used, each having flexible flanged-based tubular stemmed coupling members which fitted into each end of a coupling cylinder which passed through the hole in the plate. A tool was utilized to effect the coupling which involved eversion of the flexible stem of one of the filter cloths into place in the coupling cylinder by means of the tool. No attachment was provided for the two cloths to each other and each cloth was held in place by friction between the springy elastomeric stem and the coupling cylinder in which it resided when the filter cloth was assembled. The lack of attachment of the two cloths to each other and the varying amount of hold available from friction may be sources of leakage or provide attachment problems under rigorous conditions. Known methods for attaching the stem or coupling member to the filter cloth, such as stitching, may provide a source of leakage through the stitch holes. Manipulation of the tool to evert the coupling of one cloth, assembling the cloth on a filter plate, then withdrawing the tool may not be as rapid or efficient a method of assembly as desired.